jwt: assertion time is not updated before JWT refresh
The assertion and expiration time in the claim set do not get updated
before fetching a new JWT, hence all refreshed tokens will already be
expired.
LGTM=adg
R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/61300043
Committer: Andrew Gerrand <adg@golang.org>